Science — CBSE Class 10 board question

(a)
(i) In the diagram, the ray bends towards the normal when entering medium B from medium A. This means medium B is optically denser than medium A. Therefore, the speed of light in medium A is greater than in medium B.

(ii) The ray bends away from the normal when entering medium C from medium B. This means medium C is optically rarer than medium B. Therefore, optical density of medium B is greater than optical density of medium C.

(b) When optical density of A = optical density of C, and optical density of B > both A and C:

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Medium A → ray travels straight
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Medium B → ray bends towards normal (entering denser medium)
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Medium C → ray bends back, emerges parallel to original ray
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The emergent ray in medium C is parallel to the incident ray in medium A (similar to refraction through a glass slab).

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• Key rule: When light goes from rarer → denser medium, it bends towards the normal (speed decreases). From denser → rarer, it bends away (speed increases).
• For part (b), if density of A = density of C, the bending at the two interfaces is equal and opposite, so the emergent ray is parallel to the incident ray — this is the glass slab result from section 9.3.1.
• Examiners award marks for correct comparison AND correct ray diagram showing the parallel emergent ray.